Wireless communication systems are well known in the art. Generally, such systems comprise communication stations, which transmit and receive wireless communication signals between each other. Depending upon the type of system, communication stations typically are one of two types: base stations or wireless transmit/receive units (WTRUs), which include mobile units.
The term base station as used herein includes, but is not limited to, a base station, Node B, site controller, access point or other interfacing device in a wireless environment that provides WTRUs with wireless access to a network with which the base station is associated.
The term WTRU as used herein includes, but is not limited to, a user equipment, mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. WTRUs include personal communication devices, such as phones, video phones, and Internet ready phones that have network connections. In addition, WTRUs include portable personal computing devices, such as PDAs and notebook computers with wireless modems that have similar network capabilities. WTRUs that are portable or can otherwise change location are referred to as mobile units. Generically, base stations are also WTRUs.
Typically, a network of base stations is provided where each base station is capable of conducting concurrent wireless communications with appropriately configured WTRUs. Some WTRUs are configured to conduct wireless communications directly between each other, i.e., without being relayed through a network via a base station. This is commonly called peer-to-peer wireless communications. Where a WTRU is configured to communicate with other WTRUs it may itself be configured as and function as a base station. WTRUs can be configured for use in multiple networks with both network and peer-to-peer communications capabilities.
One type of wireless system, called a wireless local area network (WLAN), can be configured to conduct wireless communications with WTRUs equipped with WLAN modems that are also able to conduct peer-to-peer communications with similarly equipped WTRUs. Currently, WLAN modems are being integrated into many traditional communicating and computing devices by manufacturers. For example, cellular phones, personal digital assistants, and laptop computers are being built with one or more WLAN modems.
A popular wireless local area network environment with one or more WLAN base stations, typically called access points (APs), is built according to the IEEE 802.11b standard. Access to these networks usually requires user authentication procedures. Protocols for such systems are presently being standardized in the WLAN technology area. One such framework of protocols is the IEEE 802 family of standards.
The basic service set (BSS) is the basic building block of an IEEE 802.11 WLAN and this consists of WTRUs typically referred to as stations (STAs). Basically, the set of STAs which can talk to each other can form a BSS. Multiple BSSs are interconnected through an architectural component, called a distribution system (DS), to form an extended service set (ESS). An access point (AP) is a station (STA) that provides access to the DS by providing DS services and generally allows concurrent access to the DS by multiple STAs.
The 802.11 standards allow multiple transmission rates (and dynamic switching between rates) to be used to optimize throughput. The lower rates have more robust modulation characteristics that allow greater range and/or better operation in noisy environments than the higher rates. The higher rates provide better throughput. It is an optimization challenge to always select the best (highest) possible rate for any given coverage and interference condition.
The currently specified rates of various versions of the 802.11 standard are set forth in Table 1 as follows:
TABLE 1802.11 Standard Data RatesStandardSupported Rates (Mbps)802.11 (original)1, 2802.11a6, 9, 12, 18, 24, 36, 48, 54802.11b1, 2, 5.5, 11802.11g1, 2, 5.5, 6, 9, 11, 12, 18, 24, 36, 48, 54For 802.11g, the rates 6, 9, 12, 18, 24, 36, 48 and 54 Mbps use orthogonal frequency division modulation (OFDM). The choice of the rate can affect performance in terms of system and user throughput, range and fairness.
Conventionally, each 802.11 device has a Rate Control algorithm implemented in it that is controlled solely by that device. Specifically, uplink (UL) Rate Control in STAs and down link (DL) Rate Control in APs. The algorithm for rate switching is not specified by the standards. It is left up to the STA (and AP) implementation.
The inventors have recognized that each STA typically gets equal opportunity to send packet data. However, a packet sent at a lower rate takes much longer than one sent at higher rate and where a WLAN has a single shared channel, the lowest data rate will cause the capacity of the AP with which the STAs are communicating to be reduced.
Also, APs often must handle communications for multiple STAs. This presents a scheduling issue for the downlink transmissions for data to the various STAs. The inventors have recognized that data queues may be advantageously used by the APs based of on class of service in combination with the use of a priority system for releasing data from the respective queues for transmission.
In some instances, APs are configured to provide wireless services to more than one type of STA. For example, devices compliant to the IEEE 802.11g standard have become available. These devices operate in the same channels as existing 802.11b devices, but operate at a higher throughput rate. Systems operating under the 802.11g standard are preferably configured such that both 802.11b and 802.11g STAs can communicate with an 802.11g AP, in order to allow coexistence with legacy 802.11b systems.
As noted above, all 802.11 systems permit a choice of transmission rates for radio transmissions, but the rate to choose for a transmission is implementation dependent. The obvious solution is to choose the rate that maximizes throughput for a particular transmission. This implies that for the same signal strength and interference levels, 802.11g OFDM rates would always be chosen over 802.11b rates, assuming equivalent receiver performance. However, as discussed below, the inventors have recognized that this does not ensure fair access to the available bandwidth for 802.11b devices. It is thus advantageous to provide transmission rates which take into account the distinctions in the operating characteristics of 802.11b and 802.11g devices to more fairly allocate transmission rates among the 802.11b and 802.11g devices.